Bird Jacket: Pennaceous

A feather seen from under a microscope, showing the difference between insulating downy feathers and other feathers. 

Sahas Barve, a Peter Buck Fellow at the Smithsonian’s National Museum of Natural History, led a new study to examine feathers across 249 species of Himalayan songbirds, finding that birds living at higher elevations have more of the fluffy down—the type of feathers humans stuff their jackets with—than birds from lower elevations. Published on Feb. 15 in the journal Ecography, the study also finds that smaller-bodied birds, which lose heat faster than larger birds, tend to have longer feathers in proportion to their body size and thus a thicker layer of insulation.

Barve and his co-authors analyzed super-detailed photos of the feathers from the birds they studied to determine exactly how long each feather’s downy section was relative to its total length. The team was able to do that by looking at the fluffy downy section of each feather close to its base when compared to the streamlined ends of most birds’ feathers.